This invention is related to the refining of used motor, hydraulic, gear, and other oils using a membrane process as a first step and then an adsorption process as a second step to remove contaminants, such as particulates, ash and color and odorous compounds. The oil recovered through this invented process can be recycled to its original applications or reused for fuel applications.
Nationwide about 13 billion gallons of used motor and other oils are generated annually. About 70% of this is currently being recycled as fuel. The remainder is not collected and is lost in usage. Used motor oil can contain up to 15 to 20% of impurities, such as water, sludge, carbonaceous particles and oxidation products. Generally, the used motor oil contains .about.0.5 wt % of ash residue after combustion. For reuse as fuel, this ash creates air pollution concerns. Recycle of the used motor oil as a lubricant, such as crank case fluid, is possible if the contaminants are properly removed, odor is eliminated and/or the color is improved. The cleaned oil can then be reformulated with the required additives to meet specification. Presently only a small volume of the used oil is cleaned for this purpose due to (1) the significant capital investment requirement, (2) potential secondary pollution, and/or (3) substantial operating cost for re-refining the used oil.
By oil as used herein is meant to include all kinds of synthetic and mineral oils including crude oil, and particularly spent or used engine oils, hydraulic oils, cooking oils and the like from which fractions or contaminants such as debris, ash, color or odor can be removed to regenerate the oil and permit its re-use.
Most often, the used oil is an uneven product of oil collected from several sources. The main contaminants typically are:
Water from engines and storage. PA1 Dissolved gasoline and gas-oil, resulting from use in engines. PA1 Solvents, aromatics and cleaning fluids. PA1 Sediments, consisting of PA1 Lead from gasoline and anti-knock additives. PA1 Polymeric additives, for viscosity improvement or sludge dispersion (polymethacrylates, polysuccinimides). PA1 Non polymeric anti-oxidant, anti-wear or detergent-dispersing additives (zinc dialkyl-dithiophosphates, calcium or barium salicylates, alcoylphenates or sulfonates).
carbonaceous particles, resulting from bad combustion of motor fuels, PA2 metal particles, brought in by wear and corrosion, external dust.
There are several processes available for the re-refining of the used oil. The ash and other contaminants can be removed by chemical cracking, membrane-based filtration and evaporation. They are discussed as follows:
(1) Chemical Cracking
Acid treatment of the used oil has traditionally been used to flocculate carbonaceous particles and other impurities from the used motor oil. However, this process generates acid sludge, creating environmental concerns and disposal problems. Because the formulation of motor oils is sophisticated, the acid requirement increases and the product yield decreases. In addition, the handling of the concentrated acid involved in this process also presents some operation inconvenience and potential hazard. Although some color removal can be achieved along with the contaminants removal through this process, the oil treated by this process generally requires a polishing step to achieve an acceptable quality of color and smell.
(2) Membrane-based Filtration
In a Canadian patent 1,168,590 (Process for the Treatment of a Hydrocarbon Charge by High Temperature Ultrafiltration), used motor oil was treated with an Al.sub.2 O.sub.3 and MgO mixed oxide ultrafiltration membrane with pore size ranging from 50 to 250 .ANG. to remove impurities and additives. Although the used oil was reconditioned through this patented process, no color or odor removal was attempted or mentioned. Throughput from the membrane is very limited and is not practical with membrane technology available today. Moreover, the membrane is highly susceptible to fouling by the contaminants. Thus, it will be seen that the membrane-based filtration approach to cleaning spent engine oil is not without problems.
(3) Evaporation/Distillation
Evaporation/distillation of the used oil has been suggested to separate ash and other contaminants from oil. The boiling point of the used oil at atmospheric pressure is in the range of 1,020 to 1,040.degree. F. With the assistance of vacuum, temperature of 650.degree. F. was reported to effectively evaporate oil and leave the contaminants and impurities as residue. Although this process appears to be technically feasible, it is energy intense due to the phase change involved in evaporation. In addition, a polishing step is generally required to deliver an acceptable quality of color and smell.
(4) Hydrotreating
A catalytic hydrotreatment process can improve color of the used oil by destroying the undesirable oxygen and nitrogen containing compounds as well as sulfur and chlorinated species at 650.degree. F. with excess hydrogen in a pressurized vessel. Since cracking of the oil takes place in this process, a post distillation is generally required to fractionate out the desired product. Also, unused hydrogen must be scrubbed to remove HCl before recycle. Sometimes a clay adsorbent is used as a polishing step to further improve color and odor. However, this process is not economically suitable for small scale operation because of the high capital investment requirement.
(5) Clay Adsorption
Clay has been traditionally used to improve color and odor of oil. Thus far, natural clays, such as Bentonite, have been commonly used at 400-450.degree. F. to polish the treated oil from evaporation and acid cracking. These two treatment processes remove a certain degree of colors and/or odor; therefore the clay dosage requirement as a polishing step can be reduced to an economic level. About 0.3 lb/gal of the oil product was reported to reduce the treated oil from the color index of 4 to 2. Then, the clay is disposed through landfill. No regeneration of the clay has been reported.
Catalysts have been suggested to improve the color at a temperature, e.g., 300-400.degree. C., which is much higher than the standard temperature used for adsorption.